subgrades

Subgrades are the native soil or improved material that forms the foundation for pavements and railway tracks. Placed beneath the pavement structure, the subgrade supports traffic loads and transmits them to the underlying soil. The quality of the subgrade largely determines pavement performance, durability, and maintenance requirements.

Key properties include bearing capacity, stiffness (as measured by tests such as CBR or modulus of subgrade reaction), drainage, and susceptibility to moisture change. The subgrade must be capable of maintaining adequate strength with seasonal moisture fluctuations. Poorly drained or overly plastic soils can lead to excessive deformation, cracking, or frost heave.

Design and appraisal rely on in-situ and laboratory testing. Common in-situ tests are the California Bearing Ratio (CBR), plate bearing tests, and dynamic or static modulus tests; lab tests assess grain size, Atterberg limits, moisture-density relationships. Specifications often require compaction to a target dry density at a controlled moisture content, and a minimum CBR or k value for the project.

Remediation and stabilization techniques include soil stabilization with lime, cement, or fly ash, mechanical stabilization, drainage improvements, and the use of geosynthetics or drainage layers to reduce moisture rise. When the subgrade fails to meet requirements, options include excavating and replacing with better material, partial replacement, or underpinning foundations.

Subgrades are distinguished from higher pavement layers such as the subbase and base courses, and from ballast in railway construction. Their performance is influenced by climate and drainage; long-term pavement performance hinges on maintaining a suitable subgrade.